Shares of Plug Power have whiplashed dramatically since it announced a contract to supply fuel cell units to Wal-Mart in late February. After an initial rally following the “milestone” deal, shares tanked when short-seller Andrew Left of Citron Research described Plug Power as a “casino” stock and said its fair value is 50 cents.

“I don’t know how to respond to that,” said Marsh when asked about Left’s analysis. But the chief executive also pointed out that there are analysts who are just as bullish on the company as Left is bearish.

Plug Power shares, which were up 8.6% in early afternoon trading, spiked to a 49% daily gain following the publication of Marsh’s comments. Year to date, the stock is up more than 400% to $8.48, following gains of 210% in 2013.

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